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ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
time a t which the i-t behavior is more accurately described as that of a freely suspended drop, that calibration data should be obtained under the drop-time and Hg flow-rate conditions to be employed. For the purpose of calibration, the system of 1 mM Cd(I1) in 0.1 M KC1 appears to be excellent, since D for Cd(I1) under these conditions is accurately known. Considerable debate has recently centered on the questions of whether polarographic behavior at controlled drop-times of less than 1 s is accurately described by the Ilkovic equation and whether the back-pressure term in the calculation of h,-and, hence, m-is properly described by 3.1/ (mt)'I3. Canterford (37) has reviewed the various reports and has presented evidence which indicates that the Ilkovic equation does not properly describe the currents at short drop-times. The results of the present study also clearly indicate that use of a mechanical drop-knocker tends to diminish or eliminate the depletion effects, which, under conditions of natural drop-fall, counterbalance sphericity effects and permit the Ilkovic equation to describe accurately the i-h, relation. Cyclic Voltammetry. For accurate determination of the HMDE area using cyclic voltammetry or linear potential scan amperometry, a chart recorder is preferable to an oscilloscope as a read-out device because of the inherently greater accuracy of the former; however, because of the relatively slow response time of a recorder, it is generally not possible to employ scan rates exceeding 0.5 to 1 V/s; even with scan rates of ca. 0.5 V/s, low current axis sensitivities must be employed to prevent recorder response degradation, thus reducing the precision with which peak currents can be measured. (The danger associated with using too high a current axis sensitivity is exemplified by the points in Figure 6 for ul/' of 0.47 and 0.54 V1/'/sl/'.) Thus, one must evaluate the electrode area under conditions of slow scan rate, for which it is necessary t o use the cyclic voltammetric peak current equation which accounts for sphericity. The variability of the HMDE area, as indicated by the standard deviations for the peak current functions (Figures 5 and 6) is less than 3%. Because of the previously mentioned problems associated with the Cd(II)/Cd(Hg) system, this system should not be employed for HMDE area evaluation. The Fe(III)/Fe(II)
couple seems to be an excellent choice for such area evaluation.
LITERATURE CITED (1) (2) (3) (4)
J. M. Markowitz and P. J. Elving, Chem. Rev., 58, 1047 (1958). D. Ilkovic, Collect. Czech. Chem. Commun., 6. 498 (1934). J. M. Markowitz and P. J. Elving. J . Am. Chem. Soc., 81, 3518 (1959). D. M. Mohilner, J. C. Kreuser, H. Nakadomari, and P. 0. Mohilner, J . Nectrochem. Soc., 123, 359 (1976).
(5) (8) (7) (8)
G. H. Nancollas and C. A. Vincent, Electrochim. Acta, IO, 97 (1965). W. H. Reinmuth, J . Am. Chem. Soc., 79, 6358 (1957). R. S. Nicholson and I. Shain, Anal. Chem., 36, 706 (1964). T. E. Cummings, M. A. Jensen, and P. J. Elving, to be submitted for publication. H. Strehlow and M. von Stackelberg, 2 . Nekfrochem., 54, 51 (1950). M. von Stackelberg, Z . Bekfrochem., 57, 338 (1953). M. von Stackelberg and V. Toome, 2. Nektrochem., 58, 228 (1954). T. Kambara and I. Tachi, "Proc. I . Internat. Polarograph. Congress", Vol. 1, Prirodovedeche Vydavatelstvi, Prague, 1951, p 126. T. Kambara and I. Tachi, Bull. Chem. Soc. Jpn., 23, 226 (1950). R. S. Sabrahmanya, Can. J . Chem., 40, 289 (1962). H. Matsuda, Bull. Chem. SOC. Jpn., 36, 342 (1953). J. Koutecky, Czech. J . Phys., 2, 50 (1953). W. M. MacNevin and E. W. Balis, J . Am. Chem. Soc., 65, 660 (1943). G. S. Smith, Trans. Faraday Soc., 47, 63 (1952). J. W. Perram, J. E. Hayter, and R. J. Hunter, J . Elecfroanal. Chem., 42, 291 (1973). D. E.Smith, in "Electroanalytical Chemistry", Vol. 1, A. J. Bard, Ed., Dekker, New York, N.Y., 1986. J. Hevrovskv and J. Kuta. "Princioles of Polaroaraohv", Academic Press, London, 1966, p 37. C. L. Rulfs, J . Am. Chem. Soc., 76, 2071 (1954). D. J. Macero and C. L. Rulfs, J . Elecfroanal. Chem., 7, 328 (1964). M. von Stackelbera and H. Frevhold. 2.Elektrochem.. 46. 120 (1940). J. J. Lingane, Cheh. Rev., 29, 1 (1941). J. E. E. Randles and D. W. Somerton. Trans. Faraday Soc., 48, 937 (1952) R deleeuwe, M Sluyters-Rehbach, and J H Sluyters, Hectrochim. Acta, 14 1183 (1969) J. J. Lingane, J : Am. Chem. Soc., 68, 2448 (1946). D. E. Smith and W. H. Reinmuth, Anal. Chem., 33, 482 (1961). J. Kuta and I. Smoler, in "Progress in Polarography", Voi. 1, P. Zuman and I . M. Kolthoff, Ed.. Interscience, New York, N.Y., 1962, p 43. P. J. Elving and D. L. Smith, "Analytical Chemistry 1962", Elsevier, Amsterdam, 1963, pp 204-213. C. N. Reilley, G. W. Everett, and R. H. Johns, Anal. C k m . , 27, 483 (1955). P. Delahay and I. Trachtenberg, J . Am. Chem. Soc., 80, 2094 (1958). P. Delahay, J . Chim. Phys., 54, 369 (1957). J. E. E. Randles and K. W. Somerton, Trans. Faraday Sm., 48, 951 (1952). J. E. E. Randles. Faraday Soc. Discuss., 1, 11 (1947). D.R . Canterford, J . Nectroanal. Chem., 77, 113 (1977).
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) 1241 i25j (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37)
RECEIVED for review October 21, 1977. Accepted December 7,1977. The authors thank the National Science Foundation, which helped support the work described.
Solvent Extractioa of Chromium(II1) by Salicylic, Thiosalicylic, and Phthalic acids Dennis G. Sebastian' and David C. Hilderbrand" Department of Chemistry, South Dakota State University, Brookings, South Dakota 57007
The use of salicylic, thiosalicylic, and phthalic acid complexing agents for the solvent extraction of C r ( I I 1 ) from aqueous solution was investigated. N-Butanol was used as the organic solvent. The extraction efficiency was optimized with respect i o pH, heating period, choice of buffer, and concentration of a salting-out agent. An extraction efficiency of greater than 97 YO was obtained using a mixed phthalic-thiosalicylic complexing system.
Quantitative solvent extraction of many first row transition metal elements can be readily achieved a t room temperature 'Present address, Agrico Chemical Co., South Pierce Chemical Works, B a r t o w , Fla. 33830. 0003-2700/78/0350-0488$01,00/0
using a variety of complexing agents. The solvent extraction of chromium is much more difficult with quantitative extraction occurring only after extraction at elevated temperature for a prolonged period of time. One cause of chromium's poor extractability is the lability of the hexaquochromium ion. The half-life of the exchange of water molecules has been reported as 40 h (I) and corresponding rate constants for the exchange reaction of 2 X lo-' (2) to 4.8 X 10* s-' ( I ) have been reported. By comparison the rate constants for exchange of hydrated copper(I1) and iron(II1) ions are 2 X lo8 s-l and 2.5 X IO25-l (2). Chromium(II1) was chosen as the oxidation state for extraction because of its stability compared to chromium(I1). Acetylacetone, thenoyltrifluoroacetone, hydroxyquinoline, diethyldithiocarbamate, and l-phenyl-3-methyl4-benzoylpyrazolone have previously been used to extract 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
,
Table I. Effect of Heating Time on Extraction Efficiencya (mixed phthalic acid-thiosalicylic acid extraction)
~~IOSALICY~IC -
0 60-
; . I! Y
z
0.40-
P u
d 5
489
0.20-
SALICYLIC
Duration of heating, min
Extraction,
20 25 30
90.4 93.8 96.5 97.6 97.4
%
35
60
Extraction conditions where p H = 3.0 and 7.6 g NaC1/25 m L of aqueous solution. 20
30
4 0
5-0
PH
Figure 1. Relative extraction efficiency of chromium(II1) by salicylic
and thiosalicylic acids in the presence of an acetate buffer chromium at elevated temperatures (3-6). Extraction of chromium with neither salicylic, thiosalicylic, nor phthalic acids has been thoroughly studied. Salicylic acid has, however, been used in t h e extraction of trivalent ions such as aluminum(II1) and iron(II1) ( 7 ) .
Table 11. Distribution Ratios and % Extraction (mixed phthalic acid-thiosalicylic acid extraction) Extraction conditionQ
D
%E from D
5% E from recovery
20-min heating, p H 3.5 20-min heating, p H 3.0 60-min heating, pH 3.0
8.12 13.8 19.1
88.9 93.1 95.1
86.3 90.5 97.4
6.5 g NaC1/25 m L aqueous solution
EXPERIMENTAL The solvent extraction studies were performed using equal volumes of aqueous and organic solvents. The organic solvent selected was 1-butanol. Complexing agents were dissolved in the 1-butanol to provide concentrations of 0.15 M for sa!icylic and thiosalicylic acids and 0.10 M for phthalic acid. A mixed complexing agent solution that was 0.15 M in thiosalicylic acid and 0.10 M in phthalic acid was also used. The pH's of the extraction solutions were controlled by use of acetate or phthalate buffer systems. The pH of buffer-free systems was adjusted using dilute HC1 or NaOH as appropriate. The pH range investigated was from 2.0 to 5.0. The organic solvent and aqueous sample were refluxed for 2 M O min prior to separation. Sodium chloride was added to the extraction system to determine the effect of salting-out agents on the efficiency of extraction processes. After separation, the organic layer was analyzed to determine the percent extraction of chromium from the aqueous layer. For selected extractions, aliquots of both the organic and aqueous layer were analyzed for chromium to permit calculation of the distribution ratios. Solutions to be analyzed for chromium were prepared by a wet digestion procedure. Samples were initially digested with dilute nitric acid until all readily oxidizable material had been destroyed. Sample digestion was completed using a mixture of nitric, perchloric, and sulfuric acids. Chromium was determined using a Perkin-Elmer model 303 atomic absorption spectrophotometer. The spectrophotometer was equipped with a premix burner chamber and digital signal averaging unit. A fuel rich ai-acetylene flame was used to inhibit the formation of refractory oxides of chromium. From these data, percent extractions and distribution ratios were determined.
RESULTS AND DISCUSSION The relative extraction efficiencies of salicylic, thiosalicylic, and phthalic acids in the presence of different p H values and buffer systems are discussed below. Figure 1 presents the relative extraction efficiencies of salicylic and thiosalicylic acid in the presence of a sodium acetate--acetic acid buffer system. The optimum p H for the extraction of chromium(II1) with salicylic acid is 4.5 and for thiosalicylic acid is between 3.0 and 3.5. The efficiency of the extraction initially increases with increasing pH. This change is due to the higher degree of ionization of the complexing acid. After passing through a maximum, the efficiency beings to decrease. This results from the formation of nonextractable complexes that compete effectively with the complexing agent for the chromium. The p H a t which the maximum occurs is dependent on the dissociation constants of the complexing acid and the formation constants of all of the complexes. The
competing complex could be either a hydroxide complex or an acetate complex. The optimum extraction efficiency achieved using thiosalicylic acid in the presence of acetate buffer was 40% greater than for salicylic acid. Since acetate is known to form a competitive nonextractable complex with chromium(II1) (81, the extraction efficiency of chromium by thiosalicylic acid in the absence of buffer was determined. The p H of the solution to be extracted was adjusted to the desired value prior to introduction of the organic solvent-complexing agent solution. The extraction efficiency vs. p H is shown in Figure 2. The p H of the solution decreases after introduction of the complexing agent because of dissociation of the agent. The extraction efficiency achieved was greater a t high p H values than was achieved in the presence of acetate. This indicates that the acetate competes effectively with the thiosalicylic acid for the chromium at p H values where a large fraction of the acetate is in the ionized form. The extraction efficiency a t higher p H values is ultimately limited by t h e formation of hydroxide complexes. The effect of adding sodium chloride as a salting-out agent in combination with thiosalicylic acid as a complexing agent is also presented in Figure 2. The maximum extraction efficiency was increased by 20% and occurred a t a lower p H where the formation of insoluble hydroxides is less troublesome. The salt concentration was 0.2 g/mL aqueous solution. Control of p H by use of a potassium hydrogen phthalate-phthalic acid buffer system was studied as an alternative to the acetate buffer system. Phthalic acid itself is a complexing agent and was found to give extraction efficiencies in the presence of a salting-out agent similar to those obtained by use of thiosalicylic acid. When phthalic acid and thiosalicylic acid were used as the complexing agents in combination with a potassium hydrogen phthalate buffer, the highest extraction efficiencies for chromium were obtained. Addition of NaC1 further increased the extraction efficiency as shown in Figure 3. The p H range of optimum extraction efficiency was 3.0-4.0 The effect of heating time for the refluxing process on the extraction efficiency was determined. Table I indicates that the extraction efficiency was not increased by heating times exceeding 35 min. Distribution ratios and percent extractions for selected extraction conditions are presented in Table 11. Mole ratio plots for thiosalicylic and phthalic acid indicates that two moles of ligand combine with one mole of chromium.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
490
0 80-
I
. I
,
-
- L _ - L LA--__
I MOLE
-
3-5
25
4 5
Flgure 2. Relative extraction efficiency of chromium(II1)by thiosalicylic acid from an unbuffered solution in the presence and absence of a salting-out agent
-
r
